ceph: allow idmapped getattr inode op

Enable ceph_getattr() to handle idmapped mounts. This is just a matter
of passing down the mount's idmapping.

Signed-off-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>